1. Field of the Invention
The present invention relates to a transmission line switch-over control system, and in particular to a switch-over control system of a radio transmission line and a wire transmission line, or a switch-over control system of a circuit accommodated in a transmission line.
2. Description of the Related Art
In order to improve the rate of operation of a circuit accommodated in a transmission line, various switch-over schemes for switching over the circuit to its back-up transmission line when its normal transmission line has deteriorated in line quality or is disconnected have been proposed. Especially radio transmission lines have a feature that their line quality is degraded by a weather condition, such as rain or snow, although they are inexpensive and easy to install. Therefore, such a transmission line switch-over scheme as to back up the radio transmission line with a back-up transmission line is adopted.
Furthermore, in a wire transmission line such as a submarine cable as well, switch-over to a back-up transmission line is conducted when the loss for micro-bend has increased due to a twist caused by a tide on the sea bottom, or disconnection has occurred due to secular deterioration of a component in a repeater or a transmission terminal station.
FIG. 5 is a diagram for description of a conventional transmission line switch-over system, and it is a block diagram showing the configuration for switching over a telephone circuit accommodated in a radio transmission line to a back-up transmission line. The conventional transmission switch-over system includes a circuit switching system 11 for accommodating a plurality of telephone circuits 12 and conducting circuit switch-over, a radio equipment 13 for conducting modulation and demodulation of a circuit and emitting an electric wave with a radio frequency, a radio transmission line 14 serving as a normal transmission line, a back-up transmission line 15 which is a wire transmission line such as an optical cable, a line quality input data handler 16 for taking in supervisory data representing the line quality, such as a received level or BER of the radio transmission line 14, from the radio equipment 13, and a switch-over controller 17 for issuing a control signal such as a make busy signal to the circuit switching system 11 on the basis of the line quality data.
In the case where the quality of the normal transmission line is stable, speech from each telephone set is connected to the radio transmission line 14 via the circuit switching system 11 and the radio equipment 13. If the line quality is deteriorated due to a rainfall or the like and the fact that a predetermined threshold is exceeded (for example, the BER is 10.sup.-3 or less) is detected by the switch-over controller 17, however, then the switch-over controller 17 sends a switch-over signal like the Make-Busy signal (hereafter referred to as "MB signal") to the circuit switching system 11. Upon receiving the MB signal, the circuit switching system 11 conducts circuit block of the radio transmission line 14. In addition, the circuit switching system 11 conducts route alteration, and connects a new call from a telephone set to the wire transmission line 15 serving as the back-up transmission line.
In the conventional switch-over system, however, a circuit which is in conversation and connected to the circuit switching system 11 when the MB signal is received cannot be switched over to the back-up transmission line. This results in a problem that the conversation must be continued in the state of the poor line quality (with BER of 10.sup.-3 or less) until the conversation is completed. Furthermore, there is also a problem that the circuit is disconnected during the conversation in the worst case.
As one means for solving the problems, there is a method of beforehand setting a higher threshold value (for example, 10.sup.-4 or less in BER) than the poor line quality (for example, 10.sup.-3 in BER) for switching over from the normal transmission line 14 to the back-up transmission line 15.
In this method, however, circuit switch-over is conducted even in the case where the line quality is not deteriorated worse than the original threshold. Therefore, this method still has a problem that unnecessary circuit switch-over is conducted and consequently the facilities are subject to a heavier load. In addition, since in many cases a charge according to a measured rate system is made for the back-up transmission line 15, frequent use of the back-up transmission line 15 causes an unnecessary expense, resulting in poor economy.
Furthermore, in the case where an instantaneous switch-over in the lump is conducted from the whole of the normal transmission line 14 to the back-up transmission line 15 instead of the switch-over taking a circuit as the unit, the switch-over controller 17 effects the transmission line switch-over on the basis of the supervisory circuit quality data supplied from the radio equipment 13. In this case, however, there is a problem that all circuits in the conversation state are also disconnected at that time.
In the conventional transmission line switch-over system, it is impossible as heretofore described to assure the quality in conversation while avoiding unnecessary circuit and transmission switch-over and forced release of circuits. Therefore, it cannot be said that the conventional transmission line switch-over system is an effective transmission line switch-over system.